This invention relates to the preparation of a bisphenol. In one aspect, the invention relates to improving purity and yield in a process to manufacture a bisphenol employing an acidic ion exchange resin catalyst.
Bisphenols are used as the starting material in the manufacture of resins such as polycarbonate resins and epoxy resins. It is important that the bisphenol starting material is as pure as possible in order to avoid adverse effects on the properties of resulting resins.
Bisphenols can be manufactured over a strongly acidic ion exchange resin catalyst by condensation of a phenol and a ketone or an aldehyde. If a sulfonated organic polymer is used as the acidic ion exchange resin catalyst, strong organic acids such as phenol sulfonic acid have been found to leach into the reaction product mixture. As an example, typically for bisphenol-A (BPA), the product stream from the reaction zone containing BPA in solution is passed to a crystallization zone, wherein the BPA is crystallized as an adduct with phenol and the remaining solution, or "mother liquor," is recycled to the reaction zone. The leached acid will remain in the separated product stream with the crystallized BPA and cause degradation of the BPA product during subsequent process steps, particularly if thermal finishing steps are involved. It has been found that the soluble acid leached from the acidic ion exchange resin acts as a catalyst for cracking of BPA during the thermal finishing step, which results in a lower product purity and a decrease in product yield.
In order to obtain bisphenols with higher purity, it is known to use an amine-based organic anion exchange resin to remove acidic impurities from the mother liquor. Such amine-based resins are expensive and inherently less stable than the catalyst resin, and their use can result in the presence of soluble amines or the reaction products of these amines with phenol in the product stream, which will decrease product quality. When the amine-based resin is used in a recycled system, the soluble amines will in turn poison the acidic ion exchange catalyst upon recycle of unconverted reactant. Such amine-based organic resins are typically regenerated by aqueous base, which is also a poison for the acidic ion exchange resin catalyst.
It is therefore an object of the present invention to provide an acidic ion exchange resin catalyzed bisphenol preparation process with improved purity and yield. It is another object of the present invention to provide a process to remove acidic impurities from a recycled system without poisoning the acidic ion exchange resin catalyst.